Abundance and Seasonal Dynamics of Guiana Dolphins ( <scp> <i>Sotalia guianensis</i> </scp> ) in Southern Brazilian Estuarine and Adjacent Coastal Waters

ABSTRACT Estimating population parameters is crucial for understanding species dynamics and assessing anthropogenic impacts, thereby supporting conservation efforts. We evaluate key population parameters—group structure, abundance and capture–recapture probabilities—of the Guiana dolphin in the Paranaguá Estuarine Complex and adjacent coastal waters, southern Brazil. During 2021, we conducted 40 boat surveys to collect group structure and photo‐identification data for mark–recapture modelling. We sighted 317 dolphin groups, with calves present in 56% of them. Group sizes ranged from 1 to 50 individuals, including aggregations of ~100–150 dolphins in Paranaguá Bay. We identified a total of 518 individuals and estimated the abundance of Guiana dolphins using two different programs, given the sparse nature of our dataset. The programme CAPTURE estimated N = 3770 (95% CI = 2591–5483; CV = 0.19), and Program MARK estimated N = 3087 (95% CI = 2072–4599; CV = 0.21) individuals, representing the largest Guiana dolphin population reported along the species' range. Capture probabilities varied seasonally, being lower during the wet season and higher during the dry season, consistent with greater numbers of identified individuals, larger group sizes and higher calf presence during the dry season. These results highlight the influence of environmental variability on habitat use and population dynamics and underscore the importance of long‐term monitoring to understand population structure, movement patterns and anthropogenic impacts. Our findings emphasize that both biological and methodological factors must be considered when interpreting abundance estimates and that standardized approaches are essential for reliable comparisons across populations.